1. Field of Invention
The present invention relates to sensors that detect objects, such as studs, behind a wall covering, such as sheetrock, and more particularly to sensors that detect and correct a miscalibration.
2. Description of Related Art
U.S. Pat. No. 4,464,622 entitled “Electronic wall stud sensor,” issued Aug. 7, 1984, and incorporated in its entirety by reference herein, discloses an electronic wall stud sensor particularly suitable for locating a stud positioned behind a wall surface. (A “stud” is a structural member of a building to which an interior wall surface such as wall board or paneling is affixed.) Typically in the U.S., “2-by-4” wooden studs are used in construction. Nominally, a 2-by-4 stud is 51 mm (2 inches) wide and 102 mm (4 inches) deep and of any suitable length. The actual dimensions of a 2-by-4 are more typically 38 mm (1½ inches) wide and 89 mm (3½ inches) deep. Use of English units (inches) and U.S. stud sizes here is in conformance with U.S. construction practice and is not intended to be limiting, but is only illustrative. Finding studs is a typical problem for building repairs, picture hanging, etc.
The sensor detects the stud by measuring a change in capacitance due to a change in the dielectric constant along the wall. Due to the placement of the studs, a wall surface exhibits changing dielectric constants while the sensor is moved along the wall's surface. The sensor includes a plurality of capacitor plates, a circuit for detecting changes in the capacitance, and an indicator.
The plurality of capacitor plates is mounted in the sensor such that they can be positioned close to a wall's surface. In operation, an operator places a sensor over a covering surface (such as a wall, floor or ceiling). When the capacitor plates are drawn along the surface, the circuit detects a change in the capacitance of the plates due to a change in the average dielectric constant of the surface. A combination of a wall or other surface covering and an underlying stud or other member has a larger capacitance than a wall covering alone without a stud. The capacitor plates are used to measure the effective capacitance or change in capacitance of a wall.
After the sensor is placed against the wall and before detection begins, the sensor first performs a calibration to null out the effect of a wall in the absence of a stud. The sensor initially calibrates itself by determining a calibration value that may be used as a reference value. If an operator placed the sensor over an object (such as a stud or joist) hidden behind the covering surface during calibration, the sensor may detect a capacitance greater than a capacitance representing just the wall covering. As a result, the sensor may store an erroneous calibration value.
A sensor may detect the erroneous calibration value, alert the operator to begin the calibration process again, and halt the capacitance measurement process. Thus, the operator is forced to recognize the alert and know to reinitialize the calibration process by restarting the sensor over a new area of the wall covering. It may be desirable to automatically recalibrate a sensor that was erroneously calibrated over a hidden object.